In the telephone art, voice or data communications are frequently carried on over pulse code modulated transmission lines which multiplex a number of subscriber circuits onto a common PCM channel.
Between central offices, these PCM channels are carried over so-called spans consisting usually of multi-wire cables provided with repeaters at regular intervals. The repeaters maintain the pulse trains within acceptable tolerances throughout the cable. Occasionally, however, a failure occurs in one of the repeaters which renders one of the span lines unserviceable. In this event, it is desirable to temporarily switch the affected circuit or "service" span over to a spare span provided for that purpose and known as the "protective" span, with the least possible loss of PCM signal. In this manner, interruption of traffic due to the failure of the single span line in a cable can be effectively prevented.
Automatic span line switches to accomplish this result have been known for some time. As a matter of example, such a device is shown in U.S. Pat. No. 3,715,503 entitled "Automatic Transfer Arrangement for Telephone System".
The prior art span switches, though serviceable, did, however, have some significant defects. For one, they were unable to determine whether a channel had failed due to span trouble or due to trouble in the channel equipment at the terminal, in which latter case the substitution of a protective span for the affected service span would not solve the problem.
For another, prior art devices were unable to recognize the direction in which a span had failed. Consequently, maintenance personnel had to either pick an end from which to test the span and hope they picked the right one, or travel to one end of the span, loop the line, go back and test from the other end, and then return to the first end and disconnect the loop.
With an increasing number of central offices being normally unattended, a problem also arose from the fact that prior art span switches were unable to advise the nearest manned central office of a span failure between two unmanned central offices without requiring special dedicated supervisory channels for that purpose.
Although some prior art systems did have the capability of automatically resetting when the service span became operable again, the two ends of the span were not able to reset sufficiently simultaneously to prevent significant interference (in the form of an audible click or a customer-discernible loss of data) with the traffic on the affected channel.
Finally, although rudimentary priority systems have previously been known in the art, they consisted simply of grouping lines into a pair of groups, the lines of one group having priority over the lines of the other. In practice, however, it is often necessary to establish more specific orders of priority and to make at least some of these orders changeable without rewiring the frame on which the span line switch is mounted.